1. The bacterial distribution of alanine dehydrogenase (L-alanine:NAD+ oxidoreductase, deaminating, EC 1.4.1.1) was investigated, and high activity was found in Bacillus species. The enzyme has been purified to homogeneity and crystallized from B. sphaericus (IFO 3525), in which the highest activity occurs. 2. The enzyme has a molecular weight of about 230 000, and is composed of six identical subunits (Mr 38 000). 3. The enzyme acts almost specifically on L-alanine, but shows low amino-acceptor specificity; pyruvate and 2-oxobutyrate are the most preferable substrates, and 2-oxovalerate is also animated. The enzyme requires NAD+ as a cofactor, which cannot be replaced by NADP+. 4. The enzyme is stable over a wide pH range (pH 6.0--10.0), and shows maximum reactivity at approximately pH 10.5 and 9.0 for the deamination and amination reactions, respectively. 5. Alanine dehydrogenase is inhibited significantly by HgCl2, p-chloromercuribenzoate and other metals, but none of purine and pyrimidine bases, nucleosides, nucleotides, flavine compounds and pyridoxal 5'-phosphate influence the activity. 6. The reductive amination proceeds through a sequential ordered ternary-binary mechanism. NADH binds first to the enzyme followed by ammonia and pyruvate, and the products are released in the order of L-ALANINE AND NAD+. The Michaelis constants are as follows: NADH (10 microM), ammonia (28.2 mM), pyruvate (1.7 mM), L-alanine (18.9 mM) and NAD+ (0.23 mM). 7. The pro-R hydrogen at C-4 of the reduced nicotinamide ring of NADH is exclusively transferred to pyruvate; the enzyme is A-stereospecific.